We visited a geothermal power-plant, saw a large waterfall, and went to Þingvellir. It was our tourist-spot day.
The sky was overcast-to-sprinkling all day, so everyone got damp to some degree, especially during the waterfall visit. The spray was far-reaching. At the geothermal plant we got a guided explanation of the workings, and a bit of political context (controversial topics in Iceland: aluminum production and the European carbon credit system). Þingvellir had beautiful water all through it, clear and almost ice-cold, as well as informational signs on the history of the place.